Improve `EvmParams` configuration based on adding EVM

[dmitrylavrenov]

As a follow up of #214

• SplitEvmParams into EvmParams and EthereumRpcParams - with EvmParams containing only the settings required for the basic chain functionality (currently only target_gas_price), and the EthereumRpcParams to contain the RPC-related params, so that when the RPCs are not enabled these params can be omitted.

• target_gas_price should be a mandatory parameter (as in - it's not an optional value in a sense of the EVM runtime). The Evm is used as an Option outside, but this should always be present.

We can split this similarly to the changes to the cli args parser.

[dmitrylavrenov]

We need to do #253 firstly to be able to define target_gas_price as a mandatory parameter because current design doesn't allow to make it.

[MOZGIII]

This target_gas_price parameter is obsolete with #714